Tiltrotor aircraft typically include multiple propulsion assemblies that are positioned near outboard ends of a fixed wing. Each propulsion assembly may include an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a proprotor system including a rotor hub and a plurality of proprotor blades. Typically, at least a portion of each propulsion assembly is rotatable relative to the fixed wing such that the proprotor blades have a generally horizontal plane of rotation providing vertical lift for takeoff, hovering and landing, much like a conventional helicopter, and a generally vertical plane of rotation providing forward thrust for cruising in forward flight with the fixed wing providing lift, much like a conventional propeller driven airplane. In addition, tiltrotor aircraft can be operated in configurations between the helicopter flight mode and the airplane flight mode, which may be referred to as conversion flight mode.
Certain rotor systems include an articulated connection between the rotor blades and the rotor hub such that the rotor blades have three independent degrees of freedom; namely, blade pitch, blade flap and lead-lag motion. These rotor systems typically include a lead-lag damper for each rotor blade. In addition, these rotor systems may include a separate hinge for each degree of freedom of each rotor blade requiring, for example, twelve hinges in a rotor system having four rotor blades. One option for reducing the complexity of such rotor systems is to use spherical elastomeric bearings for coupling the rotor blades to the rotor hub providing a coincident hinge for all three degrees of freedom. It has been found, however, that the damping force of lead-lag dampers in such coincident hinge articulated rotor systems is affected by the pitch of the rotor blades. Such pitch dependent damping is problematic for tiltrotor aircraft as the proprotor systems must operate in both helicopter and airplane flight modes.